Lane markers, such as painted lane boundary markers (white lines) and raised markers, formed on a road work to divide the road into plural parts as lanes. Thus, in order to improve the running safety of motor vehicles, it is important for the motor vehicles in running one lane of a road ahead thereof to accurately recognize the lane markers formed on the road.
In view of the circumstances, white-line recognition systems are installed in motor vehicles. Such a white-line recognition system installed in a motor vehicle picks up an image of a region including a road ahead of the motor vehicle, and recognizes white lines (lane markers) marked on the road based on the picked-up image. One of these white-line recognition systems is disclosed in Japanese Patent Application Publication No. 2005-157670.
As one specific method for recognizing white lines marked on a road in front of a motor vehicle based on a picked-up image of a region including the road, the following recognition method is normally used. Specifically, the recognition method extracts values of light intensity (luminance) in an area of the picked-up image; this area corresponds to the region including the road. Then, the recognition method compares the values of light intensity with a preset threshold value, and recognizes white lines marked on the road based on a result of the comparison.